This invention relates to a method of manufacturing a foam molded product and more particularly a method of manufacturing a foam molded product that has at least two portions of varying densities. One specific application of this process is the manufacture of floats used in gasoline carburetors. The float is used to activate a valve or switch to allow gasoline to enter the carburetor. As the fuel in the carburetor bowl drops, the float also drops, and gasoline is allowed to enter the carburetor through a valve. As the float rises, it shuts off the flow of gasoline into the carburetor bowl.
The floats used in carburetors in the prior art are generally manufactured of a plastic material which was formed in two halves. The two halves have solid portions and other portions that form hollows so that when the two halves are placed face to face they form a float with a hollow interior. The solid portions form the hinge or other solid portions that are generally the hinge portions. The two halves are generally joined by sonically welding them to each other to create the finished float with the hollow interior and solid hinge portions. If the sonic weld or other joining means functions as intended, the seam where the two halves are joined is essentially fluid tight and free from leaks. The resultant float is intended to remain water tight, or in this case, gasoline tight and the float will remain floating in the carburetor bowl.
One problem with the prior art floats were that in the molding process that formed the two halves of the float, often times microscopic pinholes were present in the body of the float. Another problem was that the sonic welds may have not been perfect welds resulting in pinholes along the seam. If adhesives were used to join the two halves, the same problems could and did occur. This resulted in gasoline leaking into the body of the float which then caused the float to not operate properly as it became partially submerged in gasoline when it should have been floating on top of the gasoline. Another problem encountered was that depending on the material from which the float was made, the gasoline or other petrochemical substance could cause the float material to degrade which eventually caused the float to fail. This process of forming a float also required at least two steps, one to form each half and a second step in joining the two halves. Still another shortcoming of this process is that at times it required the additional step of trimming excess material from the finished product.
This invention utilizes a foam molding process used to mold polymeric materials. In the prior art, there has been used at least three different methods of foam molding of polymers. One method employs the method of injecting a gas into the tool or mold after the polymer has been injected into the tool or mold. This produces a product having a hollow center with a solid outer surface. The problem is that there is not uniformity or consistency in forming the hollow central portion and the surrounding area. Also it is not possible to form a finished product having two or more different densities that are controllable with uniformity. Throughout this application the term “tool” is meant to include both of the terms “tool” and “mold”.
Another prior art molding process that uses foam molding uses an exothermic chemical reaction that mixes an agent into the polymer during the mixing process. The mixed product is injected into the tool and allowed to expand due to the chemical reaction. Again the extent and degree of the foaming is difficult to control and the production of a part having at least two different densities is difficult, if not impossible, to control.
In another method, a gas in mixed into the polymer in the screw heating portion of the injection machine where the polymer is being melted. This produces a homogeneous mixture of the gas and polymer. This method is utilized in Applicant's process.
Applicants' invention comprises a method of manufacturing a carburetor float made of a solid, one-piece, foamed part with at least two portions having different densities. In particular the carburetor float is made using a variation of a microcellular foam injection molding technology. One such process as owned and taught by Trexel, Inc. located in Wilmington, Mass., and called the MuCell Process, involves the controlled use of gas in its supercritical state mixed with a polymer that when injected into the tool and allowed to expand, creates a foamed part. The finished part has a closed cell structure so gasoline or other petrochemical products cannot penetrate the float in a sufficient number of microscopic points such that the float will not operate properly and fail. Rather, the finished float mimics a cork like structure having numerous cells. Thus, if even a relatively large number of the surface cells fail, the ability of the float to float in the petrochemical product is not substantially diminished.
The MuCell process is good for manufacturing a homogeneous part of uniform density as the introduction of the gas into the polymeric material causes a fairly uniform dispersion of the gas throughout the product resulting in uniform expansion of the polymer and is typically limited to approximately 8%-12% reduction in weight. Applicant's inventive process results in a product having structures of at least two different densities can be made in one tool through a unique manner of injecting the polymer and gas and controlling the movement as parts of the tool and results in greatly exceeding the current expansion of 8%-12%.
The Injection Molding Process
The injection molded process used involves the highly controlled use of gas in its supercritical state to create millions of micron-sized voids in the molded part. With the correct equipment configuration, tool design, and processing conditions, these microcellular voids are relatively uniform in size and distribution.
The voids are created or nucleated as a result of homogeneous nucleation that occurs when a single-phase solution of polymer and gas (commonly nitrogen gas, but occasionally carbon dioxide) passes through the injection gate into the tool.
By using the proper ratio of the foaming material and preferably nitrogen gas, the material expands or foams within the tool to create the finished product. Applicant's inventive process controls the movement of parts of the tool, thereby allowing the polymeric material to expand or foam in parts of the tool and restricts the expansion in other parts of the tool. This results in a molded part having varying densities in different locations. Thus, one can have a part with portions having a very low density such as in the float area, and other parts having a much higher density, such as in the hinge area. Obviously the hinge area of the carburetor must be much stronger and have a much higher density than the portion of the float that is designed to float in the gasoline. Applicant's process allows the float to have at least two different densities in different parts of the finished product, while accomplishing this in a singular injection molding operation. The end result is a one piece carburetor float made of a polymer that has a floating portion with a much lower specific gravity than from the material which is originally injected into the tool and another portion having a much higher specific gravity and inherent strength in another part of the finished piece. Furthermore, the process allows the finished product to assume very intricate shapes defined by the tool cavity. This results in a float that requires little finishing, if any. The result is a substantial reduction in the cost to manufacture the product. Furthermore, the number of defective products resulting from poor sonic welding or gluing as practiced in the prior art is eliminated. Yet another advantage is that the finished part uses much less polymer than the prior molding process resulting in lower cost of production.
The foam molding process is applicable to most resins ranging from polypropylenes to nylons, polybutylene terephthalates (PBT's) and other resins known to those skilled in the art. The resin is selected from those that have the characteristics that are suited for the end product. In this particular application, the resin is selected from those having gasoline and alcohol resistance.